As an example of an inspection lighting device for use in, for example, inspection of an appearance of a product, it is possible to cite a coaxial lighting device shown in Patent Document 1, in which an imaging direction is in agreement with a lighting direction for an inspection object. This coaxial lighting device includes a light source configured to emit inspection light in a direction parallel to an inspection object surface of the inspection object, and a half mirror disposed with an inclination between the inspection object and an imaging (image pick-up) device disposed upward of the inspection object and configured to reflect the inspection light toward the inspection object and to transmit the reflected light from the inspection object toward the imaging device.
Incidentally, in recent years, there is demand for the ability to detect a feature point such as a defect that is difficult to detect even with an inspection lighting device such as that described above through a captured image thereof. More particularly, there are cases where, since a product to be inspected does not have the surface characteristics of a perfect mirror surface, precision control of the optical axis and the shape and the like of an irradiation solid angle for obtaining desired gradation information regarding a feature point on the inspection object surface is difficult, and even if the inspection light can be irradiated, significant contrast variation may occur depending on the position on the inspection object at which the feature point is present, making it difficult to identify the feature point.
For example, it is conceivable to increase the inspection precision by limiting the irradiation area of the inspection light to only the inspection object with use of an aperture stop or the like and thereby decrease stray light which is reflected light or scattered light from a non-inspection target object.
However, even when reduction of such stray light coming into the imaging device is made possible with the above-described method, in the case of a very small defect or the like, significant variation occurs in the brightness of the captured image, which makes detection thereof as a defect impossible.
More specifically, even when a small change occurs in the reflection direction of the irradiated inspection light due to the presence of, for example, a small defect on the inspection object, if this change is in such a range as confined within an observation solid angle of the imaging device, the brightness of the captured image remains unchanged, regardless of the presence/absence of the defect, or if the irradiation solid angle of the inspection light is large and the inclination of its optical axis differs at different points on the inspection object, a small change in the reflection direction cannot be captured as a change in light amount within the observation solid angle of the imaging device; moreover, the light amount within the observation solid angle of the imaging device varies irregularly between the different points on the inspection object. Consequently, the machine vision is unable to accurately detect such small defects or the like in the inspection target area.